Perchloric acid dioxolane

Perchloric acid (79% HCIO4/CH2CI2, 0°, 1 h 25°, 3 h, 87% yield) and periodic acid (aq. dioxane, 3 h, quant, yield) cleave 1,3-dioxolanes the latter drives the reaction to completion by oxidation of the ethylene glycol that forms. Yields are substantially higher from cleavage with perchloric acid (3 AHCIO4/THF, 25°, 3 h, 80% yield) than with hydrochloric acid (HCl/HOAc, 65% yield)... 

The unexpected variety found in cationic polymerizations is well illustrated by recent findings concerning the polymerization of dioxolan by perchloric acid. Gresham91 had been unable to find end-groups in polydioxolans made under anhydrous conditions and concluded that they were cyclic. His result has been confirmed by studies made with anhydrous perchloric acid as catalyst [92]. Since it is extremely unlikely that complete absence of chain ends can be achieved by a cyclization of long chains, and from other evidence, we concluded that the polymerization takes place without formation of chain ends, i.e. by a ring expansion mechanism ... 

Studies of the kinetics of the polymerisation of 1,3 dioxolan and of the depolymerisation of its polymer by perchloric acid in methylene dichloride have been accompanied by measurements of the electrical conductivity of the reaction mixtures [23]. These have shown that ions are present during the reactions and the evidence strongly suggests that they are essentially concerned in these. This is one of the very few direct demonstrations of the participation of ions. 

The next structural study of polydioxolans of DP ranging from 7 to 70 by Plesch and Westermann [6] confirmed the regular structure of the polymer. It was also shown that when a polydioxolan was formed and then depolymerised in solution by perchloric acid, the only product was monomer. This is apparently in conflict with the findings of Miki, Higashimura, and Okamura [7] who reported that a reaction mixture, in which dioxolan had been polymerised for 3 hours at 35 °C by BF3-Et20, contained 1,3,5-trioxepan, 1,4-dioxane, trioxane, and other compounds. Most probably the difference is at least partly due to the long reaction time and the use of boronfluoride, which is well known to produce more side-reactions than protonic acids. 

The object of the work described was to discriminate between the two principal rival theories concerning the polymerisation of 1,3-dioxacycloalkanes by anhydrous perchloric acid, the Mainz theory and the Keele theory . By means of Jaaks s method for determining tertiary oxonium ions we found that in polymerisations under the driest conditions the concentration of these is negligibly small. This was done with 1,3-dioxolane (1), 4-methyl-l,3-dioxolane (4), and 1,3-dioxepane (5), and the findings are supported by determinations of the content of hydroxy groups of polymers prepared and killed under different conditions. 

The generality of this title is misleading, since a worthwhile amount of information is only available for perchloric acid. The mechanism of the initiation and propagation in the polymerisation of DCA, especially 1,3-dioxolan (DXL), has been discussed many times [1-7], and the origin and nature of the differences of opinion on these subjects need not be explained here at length. The most important facts can be summarised as follows. 

It is not difficult to show by conductance measurements that when the polymerisation of dioxolan becomes of first order, roughly at the first half-life, all the available acid has reacted with monomer or oligomer to produce active centres [10]. Thus on the Mainz theory one would expect to find a number of OH groups of the same order of magnitude as the number of perchloric acid molecules introduced this is evidently not so, as long as the water concentration in the system is significantly less than that of the acid [10]. For 1,3-dioxepan the protonation is very much faster than for DXL and is complete long before the first half-life of the polymerisation. 

N-Amination of 2-[o-(l,3-dioxolan-l-yl)phenyl]pyridine (121) with O-tosylhydroxylamine gave an A-amino derivative, which underwent ring closure on the action of 70% perchloric acid to yield a pyrido[2,l-a]phthal-azinium salt (122, R = H) (92JHC1049). The 7-hydroxy derivative of a pyrido[2,l-fl]phthalalzinium salt (122, R = OH) was prepared similarly by cyclization of l-amino-2-(o-methoxycarbonylphenyl)pyridinium salt, ob-... 

The 1,3-dioxolane and 1,3-dioxane systems are stable to an extremely wide range of reaction procedures provided that an acidic medium is avoided. Deprotection is effected under a wide variety of mild acid conditions such as, for example, aqueous tartaric acid,134 perchloric acid,135 or moist silica gel either alone or in the presence of oxalic or sulphuric acids.136 The mild oxidative cleavage reaction with triphenylmethyl tetrafluoroborate in dichloromethane is... 

When acetone was used as solvent, isopropylidenation was acid dependent. Surprisingly, the reaction was slow with sulfuric acid and the selectivity was modest (entry 5). On the other hand, a large amount of p-toluenesulfonic acid (0.4 equiv) gave a 94% yield of the dioxolane 273 (0.5-2 mmol scale), but the rate, yield and selectivity of the reaction were very scale dependent (entry 6), Best results were obtained with perchloric acid together with 4 A molecular sieves to give an 89% isolated yield of the dioxolane 273 with reproducible results being obtained on a 40 mmol scale (entry 7). The minor dioxane and dioxepane products could be iso-merised to the more stable dioxolane when resubjected to the reaction conditions... 

The copolymerization between trioxane and suitable comonomers (ethylene oxide, 1,3-dioxolane, diethylene glycol formal, 1,4-butane diol formal in amounts of 2-5% by weight) is performed using cationic initiators. The cationic initiators could be Lewis acids, such as BF3 or its etherate BF3Bu20 which was used, for example by Celanese (the mechanism of this reaction was studied in detail [163,164]) or protic acids such as perchloric acid, perfluoroalkane sulfonic acids and their esters and anhydrides. Heteropoly acids were used and also a series of carbenium, oxocarbenium salts, onium compounds, and metal chelates. To regulate the molecular weight chain-transfer agents, such as methylal and butylal, are added. 

The induction periods observed in the polyrnwization of 1,3-dioxolane initiated by acids (e. g. perchloric acid trifluoromethaneailfonfc acid is apparently due to the equilibrium shifted toward much less reactive secondary oxonium ions at the early stages (short chains) of polymerization. 

The five membered cydic 1,3-dioxolane (CHjOCHjCHjO) can be polymerised by a variety of catalysts including sulphuric acid (P7), perchloric acid (98), phosphorus pentachloride (PP) and alkyl aluminium compounds with water as a co-catalyst (100). The effect of the catalyst boron trifluoride diethyl etherate on the polymerisation of 1,3-dioxolane has also been studied and it has been found that equilibrium between monomeric 1,3-dioxolane and poly(l, 3-dioxolane) is set up in both the undiluted polymer and in solution (101-104). Controverf has arisen as to whether the equilibrium is between cyclic monomer and cyclic polymer (98) or between cyclic monomer and chain polymer (104). 

Thiophen Analogues of Isoquinoline.—4,7-Dimethylthieno[2,3-c]pyridine has been synthesized by hydrolysing the acetal function of (252). This compound was obtained by the reaction of 2-(3-lithio-2-thienyl)-l,3-dioxolan with diacetyl followed by oximation and reduction. 6,7-Dimethylthieno[3,2-c]pyridine was obtained in an analogous manner, (2,5-Dimethyl-3-thienyl)acetone gave (253) on acylation. Treatment with perchloric acid gave the pyrylium salt (254), which upon reaction with NH3 gave (255). The same reaction sequence with (3-benzo-... 

Triphenyl-1,3-dithiole (233) (Rj = R2 = R3 = C6H5) has been converted into the corresponding salt by means of trityl perchlorate.24 On the other hand, the 4,5-dicyano derivative (236) [which was first prepared by Bahr et al.li8 from carbon disulfide and ammonium cyanide by spontaneous desulfurization of the salts of cyanodithio-formic acid (235), followed by ring closure with diiodomethane] is stable towards various oxidizing agents. The ease of oxidation of 233 to 234 is probably dependent on the same structural factors which govern the preparation of dioxolenium salts from 1,3-dioxolanes.149... 

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